1. Field of the Invention
The present invention is directed to class B linear-frequency response amplifiers.
2. Description of the Prior Art
Linear high-power amplifiers are required in a number of telecommunication applications. In particular, linear high power amplifiers are required in telecommunication systems operating with multiple carrier frequencies within a single radio frequency (RF) channel. Linear amplifiers are required in such systems to avoid generation of spurious signals (termed intermodulation), arising due to non-linearities in the response of the amplifier at a given carrier frequency. Such spurious signals interfere with the signals on the other carrier frequencies. Typically, such systems operate a microwave carrier frequencies in the 1-5 GHz range with 40 MH.sub.z RF channels. The ratio of the modulation frequency (carrier separation) to the carrier frequency is thus in the order of 1-2%. Such systems typically require that the amplifier have a carrier to intermodulation amplitude ratio of 20 to 30 db.
Satellite telecommunication systems have typically utilized traveling wave tube (TWT) amplifiers. However, where high carrier to intermodulation ratios are required, the efficiency of the TWT is low. A typical value of TWT efficiency for a carrier to intermodulation amplitude ratio of 30 db is in the order of 3%.
Attempts to replace TWT amplifiers with transistor amplifiers have historically failed because the efficiency of class A transistor amplifiers is unacceptably low while the modulation bandwidth of high efficiency class B or class AB transistor amplifiers have typically not been broad enough to encompass a large enough spectrum of frequencies in the RF channels, as compared to the bandwidth of TWT amplifiers. A prior art class B (or AB) transistor amplifier circuit is depicted in FIG. 1. An amplitude modulated signal is supplied by a suitable RF source 10 to the emitter 12 of an npn transistor 14, operating in the class B (or AB) mode, through a suitable transmission line 16 and DC blocking capacitor 18. Transistor 14 may be, as known in the art, either of the npn or pnp type transistor, or a field effect transistor (FET). An amplified RF signal is supplied to a load 20 from the collector 22 of transistor 14 through a suitable transmission line 24 and DC blocking capacitor 26. The base of transistor 14 is connected to ground. Such a configuration is known in the art as a common base amplifier. Collector and emitter biasing are respectively supplied by circuits generally indicated as 32 and 34. Collector biasing circuit 32 supplies to the collector 22 of transistor 14 a positive voltage with respect to ground from voltage source Vcc. An RF choke 36 and bypass capacitor 38 are provided to isolate battery Vcc from the RF signal. Emitter bias circuit 34 supplies to emitter 12 of transistor 14 a negative voltage with respect to ground from voltage source Vee. An RF choke (inductor) 40 and capacitors 42 and 44 are provided to isolate the voltage source from the RF and the modulation signal, as will be explained.
In order to operate the RF circuit in the class B mode, particular bias conditions must be provided between the emitter and ground. In addition, to maintain linearity of response, the impedance of the emitter base circuitry of transistor 14 must be maintained constant at a particular precalculated value. To accomplish this, a predetermined value of resistance, generally indicated in the art as linearization resistor 46, is inserted in the circuit between the emitter and ground. The value chosen for linearization resistor 46 is based on tradeoffs of competing design criteria. On one hand the linearization resistance must be greater than the internal emitter-base resistance of transistor 14 and greater than the reactance of inductor 40 over the range of modulation frequencies, to thereby "mask" any non-linearities and frequency related variations therein. On the other hand, the linearization resistance must be small enough so that it does not substantially reduce the gain of the amplifier. The ohmic value of the linearization resistor 46 is typically in the order of 1 ohm. Further, inductor 40 must present a negligible reactance for the entire range of modulation frequencies.
Inductor 40 embodies an inherent limitation on the range of frequencies for which the amplifier maintains a linear response. The value of inductor 40 must be such as to choke off the RF carrier signal (1-5 GHz). However, the response of the amplifier becomes non-linear with respect to those modulation frequencies high enough to cause the reactance of inductor 40 to become an appreciable factor in the impedance of the emitter-base circuit. The response of the amplifier typically becomes non-linear when the reactance of inductor 40 becomes equal to the linearization resistor 46.
In addition to maintain the impedance of the emitter-base circuitry over the entire range of modulation frequencies, the voltage sorce Vee must present an essentially resistive impedance which is negligible over the entire bandwidth with respect to the value of the total impedance of linearization resistor 46, inductor 40 and the emitter-base impedance of transistor 14. Such total impedance over the acceptable range of modulation frequencies is substantially equal to the value of linearization resistor 46. The above-mentioned common base amplifier, linearization resistor 46 and inductor 40 will hereinafter be generally referred to as amplifier 30. The prior art circuit of FIG. 1 serves to maintain a constant source impedance to a limited extent by providing a plurality of capacitors 42 and 44 across voltage source Vee in proximity to the linearization resistor 46. The values of the capacitances are chosen so that, for example, capacitor 42 operates to bypass high frequency modulation signals to ground and capacitor 44 operates to bypass low frequency modulation signals to ground. However, the bandwidth of the system with respect to the modulation signal is limited by the internal inductance of capacitor 44, generally indicated as 48. At high frequencies, the larger capacitor 44 becomes essentially inductive and a resonant circuit is formed in conjunction with smaller capacitor 42. Thus, at such resonant frequencies the impedance of the source is appreciable with respect to the value of linearization resistor 46 and the response of the amplifier becomes non-linear, causing high intermodulation distortion. Such a circuit operating at an RF carrier frequency of 4 GHz is typically linear only over the range of modulation frequencies from 0 to 3 MHz. Hence, such a circuit is not suitable as a broadband amplifier of type served by TWT amplifiers.